In recent years, development areas of energy resources such as crude oil, natural gas progress to areas of which natural environment is more severe like cold districts such as North Sea, Siberia, North America, Sakhalin, and deep oceans such as North Sea, Gulf of Mexico, Black Sea, Mediterranean Sea, Indian Ocean. Besides, a natural gas development increases from a point of view of a global environmental consideration, and at the same time, high-pressurizing of operating pressure is required from a point of view of economical efficiency of a pipeline system. Properties required for a linepipe corresponding to changes of these environmental conditions become more and more upgrading and diversified. They can be roughly classified into (a) requirement for thickening/high-strengthen, (b) requirement for high-toughness, (c) requirement for low carbon equivalent (Ceq) in accordance with improvement in field weldability, (d) requirement for high-severity of corrosion resistance, (e) requirement for high deformation properties at a frozen ground, an earthquake fault area. Besides, these properties are generally required in a composite manner in accordance with a usage environment thereof.
Further, development at a distant place, an area in severe natural environment which are left as it is from a point of view of profitability, begins to fully start on the back of recent increasing demand of the crude oil and the natural gas. In particular, the high-toughness capable of being used at the cold district in addition to the thickening, the high-strength to improve transport efficiency are strongly required for a linepipe used for a pipeline in which the crude oil and the natural gas are transported for a long distance, and it is a technical problem to enable both of these required properties.